The subject matter disclosed herein relates generally to process control systems.
Flow controls and process devices can be used on process lines that transfer and distribute materials (e.g., gas and liquid) in the chemical industry, refining industry, oil & gas recovery industry, and the like. Valve assemblies are a type of process device that can regulate a flow of these materials. These process devices can include a pneumatic actuator that couples with a closure member (via a stem). The closure member may embody a plug, a ball, a butterfly valve, and/or like implement that can contact a seat to prevent flow. Valve assemblies can also be configured with a sensing mechanism to monitor the position of the closure member. This sensing mechanism can have a position sensor and a mechanical linkage that couples the position sensor with the stem or other structure that moves in concert with the closure member. Valve assemblies can also include a valve positioner that couples with the position sensor. The valve positioner can be configured to regulate instrument gas into the actuator. The instrument gas can pressurize (and de-pressurize) the actuator in order to locate the closure member in position relative to the seat.
During operation, the valve positioner can receive and process a control signal from a controller that is part of a process control system (also “distributed control system” or “DCS system”). The control signal may define operating parameters for the valve assembly in accordance with an appropriate flow of fluid from the valve assembly and into the process line. The valve positioner can use these operating parameters and the input from the position sensor to regulate the instrument gas to set the position required for the closure member to achieve the appropriate flow.
In process control systems, accuracy of the process devices is critical to achieve appropriate performance of the process line. Valve assemblies are no exception. As noted above, the accuracy of these devices relies, at least in part, on mechanical sensing mechanisms to monitor the position of the closure member. Like all mechanical assemblies, however, values from the sensing mechanism may change during the operating life of the valve assembly on the process line. These changes can introduces certain errors that can frustrate the accuracy of the device.
Accuracy issues do not pervade all types of valve assemblies used in the process control industry. For example, changes in the sensing mechanism are negligible in applications in which the valve assembly and the process control system rely on analog devices and signals. On the other hand, valve assemblies that leverage digital technology (e.g., microprocessors) are more likely susceptible to problems because these valve assemblies often use the input value from the position sensor to calculate the position for the closure member. Thus, any change in the input value has the potential to induce an error in the calculated position.
Unfortunately, there are few opportunities to adjust the sensing mechanism to correct these errors once the process begins on the process line. Many plants and factories restrict access to any process device once the process is operating at its desired parameters. These restrictions are meant to avoid unnecessary disruptions that can lower yields, reduce quality, and result in significant costs and wasted product. In practice, most maintenance occurs during plant shut-downs to avoid the need to take the process line (or any part thereof) down or off-line. Other solutions to correct errors involve “hot swapping” the valve assembly as the process runs, often by locking the position of the closure member and replacing the valve positioner with another valve positioner that is configured to address problems with the mechanical linkage to improve accuracy of the position of the closure member during operation of the valve assembly.